Due to current efforts to reduce consumption of natural resources, the conversion of wind energy to electrical energy using wind turbine generators is becoming more prevalent. Wind turbines exploit wind energy by converting the wind energy to electricity for distribution to end users.
A fixed-speed wind turbine is typically connected to the grid through an induction (asynchronous) generator for generating real power. Wind-driven blades drive a rotor of a fixed-speed wind turbine that in turn operates through a gear box (i.e., a transmission) with a fixed rotational speed output. The fixed-speed gear box output is connected to an induction generator for generating real power.
The rotor and its conductors rotate faster than the rotating flux applied to the stator from the grid (i.e., higher than the synchronous field frequency). At this higher speed, the direction of the rotor current is reversed, in turn reversing the counter EMF generated in the rotor windings, and by generator action (induction) causing current (and real power) to be generated in and flow from the stator windings.
The frequency of the generated stator voltage is the same as the frequency of the applied stator voltage providing the excitation. The induction generator may use a capacitor bank for reducing reactive power consumption (i.e., the power required to generate the stator flux) from the power system.
The fixed-speed wind turbine is simple, reliable, low-cost and proven. But its disadvantages include uncontrollable reactive power consumption (as required to generate the stator rotating flux), mechanical stresses, limited control of power quality and relatively inefficient operation. In fact, wind speed fluctuations result in mechanical torque fluctuations that then result in fluctuations in the electrical power on the grid.
In contrast to a fixed-speed wind turbine, the rotational speed of a variable speed wind turbine can continuously adapt to the wind speed, with the blade speed maintained at a relatively constant value corresponding to a maximum electrical power output through the use of a gear box disposed between the wind turbine rotor and the generator rotor.
The variable speed wind turbine may be of a doubly-fed induction generator (DFIG) design or a full converter design. The doubly-fed induction generator uses a partial converter to interchange power between the wound induction generator rotor and the power system. The full converter wind turbine is typically equipped with a synchronous or asynchronous generator (the output of which is a variable frequency AC based on the wind speed) and connected to the grid through a power converter that rectifies the incoming variable-frequency AC to DC and inverts the DC to a fixed-frequency 60 Hz AC. Variable-speed wind turbines have become widespread due to their increased efficiency over fixed-speed wind turbines and superior ancillary service capabilities.
FIG.1 illustrates components of an exemplary variable speed wind turbine 8, including rotor blades 12 for converting wind energy to rotational energy for driving a shaft 16 connected to a gearbox 18. The wind turbine also includes a structural support component, such as a tower and a rotor pointing mechanism, not shown in FIG.1. The gearbox 18 converts low speed rotation to high speed rotation, as required for driving a generator 20 to generate electricity.
Electricity generated by the generator 20 is supplied to a power electronics system 24 to adjust the generator output voltage and/or frequency for supply to a grid 28 via a step-up transformer 30. The low-voltage side of the transformer is connected to the power electronics system 24 and the high-voltage side to the grid 28. Generally, the power electronics system imparts characteristics to the generated electricity that are required to match electricity flowing on the grid, including controllable active power flow and voltage regulation and improved network voltage stability.
One embodiment of the power electronics system 24 includes a generator-side converter for converting the generated AC electricity to DC and an output capacitor for filtering the DC current. DC current is supplied to a line side converter (inverter) for producing 60 Hz AC power supplied to the grid 28. The amount of power available from the wind turbine is determined by operation of the generator-side converter.
One type of converter employed in a variable speed wind turbine, referred to as a full converter or a back-to-back converter, comprises a power converter connected to the generator side, a DC link and a power converter connected to the grid. The full converter converts an input voltage, i.e., a fixed frequency alternating current, a variable frequency alternating current (due to the variable wind speed) or a direct current, as generated by the wind turbine, to a desired output frequency and voltage as determined by the grid that it supplies. Typically using thryistors, the full converter converts the electricity produced by the generator to DC and transfers this energy to the DC link.
From the DC link the electricity is supplied to the grid-side active converter where it is transformed to fixed frequency AC electricity and supplied to the grid.
FIG.2 illustrates a wind park or wind farm 50 comprising a plurality of wind turbines 54 (such as the variable speed wind turbine 8 illustrated in FIG.1 or a fixed speed wind turbine) connected through a feeder or collector 56, which serves as a distribution system within the wind turbine park. Several feeders may be required for an average size wind turbine park.
The wind park 50 further comprises a wind park controller 60 and a wind park transformer 64. The wind park controller 60 controls operation of the wind turbines 54. The transformer 64 connects the wind park collector 56 to a utility system or grid 68 via a point of common coupling (PCC) 72.
The wind turbines 54 bidirectionally communicate with the controller 60 via control lines 78. The signals carried over the control lines 78 relate to wind turbine output power, wind turbine status, a reference power, wind turbine operational commands, etc. The controller 60 is also connected to the PCC 72 via a control line 80. This connection allows the controller 60 to detect power parameters, such as voltage and current, at the PCC 72.
The wind park controller 60 generally fulfills a plurality of control functions related to the individual wind turbines 54 and therefore the output of the wind park 50. For example, the wind park controller 60 collects data characterizing the current state of each wind turbine 54 and in response thereto independently controls operation of each wind turbine 54.
The wind park 50 is only an example of a conventional wind turbine park. The teachings of the present are not restricted to the depicted layout of FIG.2.